It was reported that the yield in Akenohoshi (a high yielding rice cultivar) was about 1780 kg/ha higher than that in Nipponbare (a Japanese standard cultivar), and that Akenohoshi had a higher dry matter production capacity compared with Nipponbare. This was attributed to the maintenance of a higher net assimilation rate during the ripening period in the former. The higher net assimilation rate of Akenohoshi resulted from a slower leaf senescence than that of Nipponbare at the ripening stage. To clarify the cause of the slower senescence in Akenohoshi, changes in exudation rate and cytokinin activities in the exudates were measured in two varieties of rice. The exudation rates of Akenohoshi, the leaves of which remained green for a longer time, were higher than those of Nipponbare after the booting stage. Cytokinin activities in the exudates of Akenohoshi were higher than those of Nipponbare during the ripening period. Cytokinins in the exudates collected during the middle of the ripening stage were analyzed with mass spectrometry using deuterium-labeled standards. trans-Zeatin, trans-ribosylzeatin, and N^6-isopentenyladenosine were detected in the hydrolysates of highly polar fractions("conjugated zeatin") in the exudates of both cultivars. Therefore, we suggest that conjugated zeatin is an important form of cytokinin during the ripening stage. The level of each of the cytokinins in Akenohoshi was higher than that in Nipponbare.